Although it has been established that the use of estrogens by post- menopausal women increases their risk of endometrial cancer, hormone replacement therapy is still widely prescribed by gynecologists. Frequently progestins are prescribed in combination with estrogens, since pathological studies have demonstrated the beneficial effect of progestin in reversing histological changes caused by estrogens. However, there have been no convincing epidemiological studies to date which have quantified the influence of combined estrogen-progestin therapy on the incidence of endometrial cancer. The proposed study is designed to: 1. Estimate the risk of endometrial cancer associated with combined estrogen pius progestogen therapy, relative both to no therapy, and to estrogen only therapy in postmenopausal women. 2. Determine if this risk is further influenced by the number of days in the cycle on which the progestogen is given in addition to estrogen. 3. Determine the length of the estrogen-free interval after which the risk of endometrial cancer approaches background rates, for ifferent durations of prior estrogen use. case-control study of about 660 newly diagnosed cases of endometrial cancer, identified through a population-based cancer registry and confirmed by independent histological review, and about 660 controls, recruited using the technique of random digit dialing, will be conducted in three counties surrounding Seattle, Washington. Information on past hormone use will be obtained via personal interview, using a pictorial display of estrogen & progestin preparations as a memory aid. Cases and controls will be compared on history of past hormone use, taking into account potential confounding factors such as age, race, parity, obesity and dietary factors using logistic regression techniques.